The present invention relates to three-stage surface acoustic wave filters of the ladder type comprising three serial resonators connected to a serial line and three parallel resonators connected to parallel lines.
Surface acoustic wave filters are conventionally used as resonator filters in communications devices such as portable telephones.
For example, FIG. 3 shows a surface acoustic wave filter comprising three serial resonators 6a, 6b, 6c connected to a serial line 4 of a ladder-type circuit and three parallel resonators 7a, 7b, 7c connected to respective parallel lines 5 of the circuit. The three serial resonators 6a, 6b, 6c have respective resonance frequencies frs1, frs2, frs3 which are of the same value, and the three parallel resonators 7a, 7b, 7c have respective resonance frequencies frp1, frp2, frp3 which are of the same value. The resonance frequency differences xcex94f1, xcex94f2, xcex94f3 between the resonance frequencies of the serial resonators and those of the corresponding parallel resonators are of the same value.
As shown in FIG. 4, each of the serial resonators 6a, 6b, 6c comprises an interdigital electrode 2 and grating reflectors 3, 3 which are formed on the surface of a piezoelectric substrate 1. The parallel resonators 7a, 7b, 7c are identical with the serial resonators 6a, 6b, 6c in construction.
In recent years, there is a demand for surface acoustic wave filters having a greater pass band width for use in portable telephones.
Accordingly it is attempted to give a wider pass band by increasing the resonance frequency differences xcex94f1xcex94f2, xcex94f3 between the resonance frequencies frs1, frs2, frs3 of the serial resonators 6a, 6b, 6c shown in FIG. 3 and the resonance frequencies frp1, frp2, frp3 of the corresponding parallel resonators 7a, 7b, 7c shown.
However, setting the three resonance frequency differences at an increased value produces adverse effects on the characteristics of the surface acoustic wave filter as will be described below.
FIG. 5 shows the VSWR (voltage standing wave ratio) and insertion loss characteristics of surface acoustic wave filters at varying frequencies. The VSWR-frequency characteristics and insertion loss-frequency characteristics of a surface acoustic wave filter having a pass band of relatively small width are represented by a thin solid line and thick solid line, respectively. The VSWR-frequency characteristics and insertion loss-frequency characteristics of a surface acoustic wave filter having increased resonance frequency differences are represented by a thin broken line and thick broken line, respectively. It is desired that the VSWR be small in the pass band.
As indicated by an arrow (1), the filter having increased resonance frequency differences is greater in VSWR within the pass band than the filter having a small pass band width.
The filter of small pass band width exhibits an insertion loss characteristics curve which is generally flat and relatively small in ripple within the pass band, while the filter of great resonance frequency differences exhibits an insertion loss characteristics curve which has a furrow around the center frequency f0 as indicated by an arrow (2) and is increased in ripple value.
Further the insertion loss characteristics curve of the filter of small pass band width has an approximately constant great slope in the entire boundary frequency band between the pass band and the lower-frequency outer side thereof, exhibiting satisfactory cut-off characteristics, whereas the insertion loss characteristics curve of the filter of great resonance frequency differences has a gentle slope at a portion of the boundary frequency band as indicated by an arrow (3), hence impaired cut-off characteristics.
In order to obtain a filter which is widened in pass band while retaining satisfactory characteristics comparable to those of the conventional surface acoustic wave filter of small pass band width, a method is used of optimizing parameters such as the above-mentioned resonance frequency differences, the capacitances of the resonators which are dependent on the number of pair of the electrode fingers and aperture width of each resonator, and the capacitance ratio of the serial resonators to the parallel resonators (JP-A No. 340232/1996 and JP-A No. 340234/1996).
However, this method of optimizing the resonance frequency differences, capacitances of the resonators, etc. involves many parameters which need to be adjusted, requires repeated fabrication of a large number of filters for experiments and therefore has the problem of necessitating much time and labor before a surface acoustic wave filter of desired performance is obtained.
The surface acoustic wave filter obtained by the above method can be reduced to not greater than specified values in insertion loss, ripple and VSWR value within the pass band, whereas the filter involves the problem that the insertion loss characteristics curve thereof has a gentle slope in a portion of the boundary frequency band between the pass band and the lower-frequency outer side thereof, hence impaired cut-off characteristics and unsatisfactory filter characteristics.
An object of the present invention is to provide a three-stage surface acoustic wave filter of the ladder type which is widened in pass band while reliably retaining satisfactory characteristics comparable to those of the conventional surface acoustic wave filter of small pass band width, and to reduce the time and labor required for fabricating the desired filter having such characteristics.
Accordingly, we have conducted intensive research to fulfill the above object and accomplished the present invention based on the finding that making the stages of the filter different in the resonance frequency difference between the resonance frequency of the serial resonator of the stage and the resonance frequency of the parallel resonator thereof influences the width of the pass band and characteristics of the filter.
The present invention provides a three-stage surface acoustic wave filter of the ladder type which comprises three serial resonators connected to a serial line and three parallel resonators connected to parallel lines, and wherein the resonance frequency difference xcex94f2 between the resonance frequency frs2 of the serial resonator in the intermediate stage and the resonance frequency frp2 of the parallel resonator in the intermediate stage is set at a value greater than the resonance frequency difference xcex94f1 (xcex94f3) between the resonance frequency frs1 (frs3 ) of the serial resonator in each of the front and rear stages and the resonance frequency frp1 (frp3) of the parallel resonator therein.
With the surface acoustic wave filter of the present invention, the resonance frequency difference xcex94f2 of the intermediate stage is made greater than the resonance frequency difference xcex94f1 (xcex94f3) of each of the front and rear stages, whereby the filter is given a greater pass bandwidth than in the prior art. The filter of the present invention has a VSWR reduced to as small a value as in the prior art within the pass band and an insertion loss characteristics curve which is generally flat in the pass band with ripple suppressed to as small a value as in the prior art and which has an approximately constant great slope in the entire boundary frequency band between the pass band and the outside thereof, hence satisfactory cut-off characteristics. Thus, the filter of the invention reliably retains satisfactory characteristics comparable to those of the conventional surface acoustic wave filter of small pass band width. Experiments have substantiated that the present filter has a greater pass band width than the conventional filter and is as satisfactory as the conventional one in characteristics.
The parameters which need to be adjusted to provide the filter of the invention thus constructed are limited to six, i.e., the resonance frequencies frs1 to frs3 and frp1 to frp3 of the serial and parallel resonators. This reduces the number of filters to be fabricated for experiments unlike the conventional method described which requires the adjustment of parameters, such as the capacitances of the resonators and capacitance ratio, other than the resonance frequencies of the resonators. As a result, the desired surface acoustic wave filter is available within a shorter period of time and with reduced labor.
Stated more specifically, the ratio of the resonance frequency difference xcex94F between the resonance frequency difference xcex94f2 of the intermediate stage and the resonance frequency difference xcex94f1 (xcex94f3 )of each of the front and rear stages to the center frequency f0 (hereinafter referred to merely as the xe2x80x9cfrequency difference ratioxe2x80x9d) should be in the range defined by Mathematical Expression 1 given below.
0 less than xcex94F/f0 less than 0.02xe2x80x83xe2x80x83(Mathematical Expression 1)
When the frequency difference ratio is greater than 0, the filter can be given a wider pass band while retaining characteristics comparable to those of the conventional filter.
If the frequency difference ratio is not smaller than 0.02, on the other hand, the insertion loss outside the pass band diminishes to result in lower ability to suppress noise.
It is therefore desired that the frequency difference ratio be in the above range.
More preferably, the frequency difference ratio should be a value defined by Mathematical Expression 2 given below.
xcex94F/f0=0.01xe2x80x83xe2x80x83(Mathematical Expression 2)
According to the present invention, a three-stage surface acoustic wave filter of the ladder type is available which is given a widened pass band while reliably retaining satisfactory characteristics comparable to those of the conventional surface acoustic wave filter of small pass band width.
Furthermore, the invention reduces the time and labor required for obtaining the desired surface acoustic wave filter exhibiting such performance.